Process for producing a grain-oriented electrical steel sheet having excellent magnetic characteristic

ABSTRACT

A process for producing a high magnetic flux density grainoriented electrical steel sheet having at least more than 1.85 Wb/m2 by subjecting a silicon steel containing not less than 2% Si, 0.010 to 0.045% sol.Al, 0.0030 to 0.0080% total N, and unavoidable impurities to a light hot working immediately after the hot rolling or prior to the cold rolling to precipitate AlN.

United States Patent Watanabe et a1.

PROCESS FOR PRODUCING A GRAIN-ORIENTED ELECTRICAL STEEL SHEET HAVING EXCELLENT MAGNETIC CHARACTERISTIC Inventors: Shozo Watanabe, Tokyo; Kiyoshi Tanaka; Yozo Suga, both of Himeji,

all of Japan Assignee: Nippon Steel Corporation, Japan Filed: Oct. 3, 1973 Appl. N1: 402,908

Foreign Application Priority Data Oct. 11, 1972 Japa: 47101795 US. Cl. 148/111; 148/3155; 148/112;

148/120, 148/121 Int. Cl. H011 l/04 Field of Search 148/111, 112, 110, 120,

[451 July 22, 1975 [56] References Cited UNITED STATES PATENTS 2,084,337 6/1937 Goss 148/111 3,144,363 8/1964 Aspden ct alum. 148/111 3,671,337 6/1972 Kumai et a1. 1411/1 11 3,764,406 10/1973 Littmann 148/1 1 1 Primary E.raminer-Walter R. Satterfield Attorney, Agent, or Firm-Toren, McGeady and Stanger [57] ABSTRACT 2 Claims, 6 Drawing Figures PATENTEDJUL22 I975 3 974 SITE 1 F I G I HOLDING TIME REDUCTION RATE I% F I G 2 HOLDING TIME "95 I 4 SEC.

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BENDING TEMPERATURE C) PATENTEDJUL22 ms 1 1,895,974 SHEET 2 F I G 3 L90 O: .0. I. 0

BB 00 on I0 L80 y z) REDUCTION RATE 9.5 -|0.5% TEMPERATURE 800C L75 I lb 2 0 3 0 50 7 0 I00 HOLDING TIME (SEC) PROCESS FOR PRODUCING A GRAIN-ORIENTED ELECTRICAL STEEL SHEET HAVING EXCELLENT MAGNETIC CHARACTERISTIC The present invention relates to a process for producing so-called grain-oriented electrical steel sheets having a magnetizable axis l in the rolling direction.

Silicon steels widely used presently as materials for the grain-oriented electrical steel sheets are composed of grains having a body-centred cubic lattice, and it is well known that the direction 100 along the three tetragonal axis is most easily magnetized. The grainoriented electrical steel sheet is a steel sheet in which this magnetizable axis l 00 is arranged in the rolling direction and the plane {110} is arranged in parallel to the rolling plane, and crystallographically it is referred to as (ll0)[00l] orientation by the Miller indices.

As above explained, the grain-oriented electrical steel sheet is composed of the grains having the specific selective orientation, and such grain orientation is attained by the secondary recrystallization phenomenon during the final annealing of the cold rolled steel sheet of final thickness.

The grain-oriented electrical steel sheets are used mainly as iron cores for transformers and other electrical appliances, and they must have good exitation and low watt loss characteristics as magnetic properties.

In the present invention, as the value representing the exitation characteristics, the magnetic flux density B (Wb/m produced in the iron core at an intensity of the magnetic field H 800A'T/m (ampere per meter) is used, and the value for representing the energy loss caused when predetermined A.C magnetic flux density is given to the iron core, namely the watt loss, the watt loss at A.C magnetic flux density of 1.7 Wb/m and 50 Hz frequency is used.

In recent years, tendencies have been toward enlarged sizes of the electrical appliances and trials have been made for increasing the magnetic flux density value and for decreasing the weight of the iron core in view of the restrictions in transportion and others. For this purpose, an electrical steel sheet showing excellent magnetic characteristics in a high magnetic flux density zone has been demanded.

Therefore one of the objects of the present invention is to provide a process for producing an electrical sheel sheet which satisfies the above requirements.

According to the present invention, it is possible to produce a high magnetic flux density grain-oriented electrical steel sheet which shows a magnetic flux density B in the rolling direction of at least more than 1.85 Wb/m More particularly, the present invention is to provide a process for producing grain-oriented electrical steel sheet having excellent magnetic characteristics by hot rolling a silicon steel material comprising not less than 2% preferably 2.0 to 4.5% silicon, 0.010 to 0.045% preferably 0.027 to 0.045% acid soluble aluminium (soLAl), 0.0030 to 0.0095% preferably 0.0065 to 0.0095% nitrogen, with the balance being iron and unavoidable impurities, cold rolling the hot rolled material to the final sheet thickness and subjecting the cold rolled sheet to a high temperature annealing, which is characterized in that the hot rolled material is subjected to light hot working of 3 to 20% preferably to 16% at a temperature between 700 and 900C preferably 720 and 830C and held in the temperature Table 1 Elements Contents C 0.025 0.060% Si more than 2% Mn less than 0.15%

P less than 0.025% S less than 0.030% SoLAl 0.0l0 0.045%

As the present invention is directed to obtain a high grade electrical steel sheet having excellent magnetic properties at a high magnetic field, the silicon content is defined as not less than 2%.

As the dispersed precipitates utilized to obtain grainoriented electrical sheets through the secondary recrystallization in the present invention. MN is mainly utilized, and thus the amounts of acid soluble aluminium and total nitrogen which form AlN are particularly defined.

Regarding the content of acid soluble aluminium. with less than 0.010% acid soluble aluminium, the amount of AlN is not enough to be utilized as the dispersed precipitates. On the other hand, if the content of acid soluble aluminium is beyond 0.045%, uniformity in the refinement of the dispersed precipitates of AIN is damaged. Therefore the content of acid soluble aluminium is limited to 0.010 to 0.045%.

As for the content of the total nitrogen, if the total nitrogen is less than 0.0030%, the amount of MN is not enough as in the case of the acid soluble aluminium and if the total nitrogen is over 0.0095%, there will be caused in the final product surface defects such as commonly called blisters. Therefore, the total nitrogen content is limited to 0.0030 to 0.0095%.

According to the present invention, the silicon steel as shown in Table l is used as starting material, and is subjected to a light hot working immediately after the hot rolling or prior to the cold rolling to obtain appropriate precipitation of AlN, thereby a high magnetic flux density grain-oriented electrical steel sheet having at least more than 1.85 \Vb/m of flux density B can be produced.

it is disclosed in a Japanese Pat. No. Sho 4045644 to utilize AlN as the dispersed precipitate for the secondary recrystallization and to adjust the precipitation of AlN prior to the cold rolling for the production of a high magnetic flux density grain-oriented electrical steel sheet.

However, the precipitation treatment of AlN in the above prior art is conducted by a high temperature heat treatment between 900 and l,200C for 30 seconds to 30 minutes. This is completely different from the present invention in which the precipitation of AlN is made uniform by the light hot working.

According to the present invention, the light hot working is conducted in a temperature range in which the precipitation of AlN takes place suddenly to disperse the precipitation sites thereby finely and uniformly dispersed precipitates of AlN most suitable to the secondary recrystallization is rapidly obtained. Thus the present invention is based on technical thought completely different from the precipitation treatment of AlN conducted after the light cold working, or the simple precipitation treatment of AlN without a light working.

Further. in order to conduct the precipitation treatment of AlN according to the prior art Japanese Pat. No. Sho 40-15644. a heat treatment equipment capable to conduct the high temperature heat treatment is required, whereas in the present invention, the MN precipitation conditions can be obtained immediately after the continuous hot rolling only by adding special conditions in the ordinary continuous hot rolling mill. Thus the present invention is advantageous in economical aspects, too.

The present invention will be described in detail by referring to the attached drawings.

FlG. l is a graph showing the relation among the light hot rolling temperature the rolling reduction rate and the 8, value.

FIG. 2 is a graph showing the relation among the bending ratio, the bending temperature and the B value.

FIG. 3 is a graph showing the relation between the holding time after the light hot working and the B value.

FIG. 4 to FIGv 6 are electron microscopic photographs showing the precipitation of AlN.

FIG. 1 shows the magnetic flux density ofa specimen which was prepared by heating rapidly hot rolled steel plate of 2.3mm thickness containing 2.95% Si, 0.025% soLAl. 0.0058% total N to a temperature between 700 and l,lC, giving the plate a light rolling of 0 to about 26%. holding the plate for 4 seconds, quenching the plate in a salt water, cold rolling the plate to 030mm thickness, decarburizing the sheet in a wet hydrogen atmosphere. and subjecting the sheet to a high temperature annealing for the secondary recrystallization.

As clear from FIG. 1, a grain-oriented electrical steel sheet having high magnetic flux density can be obtained by conducting the light hot working at a temper ature between 700 and l.l00C with a reduction rate between 3 and and the light hot working conditions of the present invention have been defined on the basis of the above experiments.

The results shown in FIG. 1 were obtained by adopting the hot rolling as the light hot working, but the light hot working can be conducted by any of other equipments which can give the specified reduction rate such as roller levellers, stretcher levellers and so on.

For example, FIG. 2 shows the magnetic flux density of a specimen prepared by rapidly heating the hot rolled steel sheet as used in FIG. 1 to a temperature between 500 and 1,000C, bending the sheet with bending radius of mm and 50mm, holding the sheet for 4 seconds, quenching the sheet in a salt water and then treating the sheet in a similar way as in HO 1.

The maximum working ratios corresponding to the bending ratios of 25mm and 50mm are 4.4% and 2.3% respectively, and it is understood that the desired re sults of the present invention can be obtained if the temperature and the working ratio for the light hot working are satisfied irrespective to the kind of the light hot working.

The other feature of the present invention is to obtain an appropriate precipitation of AlN by the light hot working, and for this purpose a certain holding time is required for changing the AlN precipitation.

FIG. 3 shows magnetic flux densities of five specimens prepared by rapidly heating to 800C five hot rolled steel sheets of 2.3mm thickness containing 2.5 to 3.2% Si. 0.015 to 0.025% sol.Al, and 0.0050 to 0.0070% total N, subjecting the sheets to light hot rolling of 9.5 to 10.5%, holding the sheets at various times after the light hot rolling to the quenching in a salt water (the temperature of the sheets after the light hot rolling was about 750C), quenching the sheets in a salt water, cold rolling the sheets to 030mm thickness and subjecting the sheets to decarburization annealing and high temperature annealing. The results show that the object of the present invention is attained by the holding times not shorter than 2 seconds.

Further, according to the present invention, MN is utilized as the dispersed precipitates for the secondary recrystallization and the light hot working is utilized for rendering the AlN precipitates into an appropriate form. The present inventors have conducted studies on effects of the light hot working on the form of AlN precipitates.

Samples were taken from the specimens used in FIG. 1 at each of the steps and subjected to chemical analysis and electron microscope observation.

Table 2 Changes of AlN Figures in Table 2 show N as AIN (amount of N precipitated as AIN). Unit: HYQE As shown in Table 2, the amount of AlN in the hot rolled steel sheets remarkably increases when the sheets were heated to respective temperatures irrespective to the light hot working. (ln actual, the amount is slightly larger when the light hot rolling is done.)

However, according to the observation by the electron microscope, clear differences are observed in the AlN precipitates between the sheet subjected to the light hot working and the one subjected to no such working.

Namely, FIG. 4 shows the distribution of AlN in the hot rolled sheet, FIG. 5 shows the sheet which was heated rapidly to 800C and cooled without the light hot rolling, and FIG. 6 shows AIN in the sheet which was heated rapidly to 800C and subjected to light hot rolling of 11%.

As is understood from the comparison of FIG. 4 to FIG. 6, MN precipitates finely and uniformly when the light hot rolling is conducted. On the other hand, when the light hot rolling is not conducted, the AlN precipitates are in flocks, resulting in irregular distribution of AlN.

Therefore, it is assumed a grain-oriented electrical steel sheet having a high magnetic density flux can be EXAMPLE 2 The hot rolled steel strips obtained by the conventional method and used in Example I were subjected to a light hot rolling as shown in Table 5. cold rolled and Oblamed by the llght hot Grklng the p 'f mven' sub ected to decarburizlng annealing and a high tem- IIOD due to the fact that the and l-mlfgrmly P perature annealing to obtain electrical steel sheets of cipitated AlN formed by the light hot working has re- 030 hi k markable effects on the cold rolling or on the primary Table: 5 recrystallization structure or on the secondary recrysm tanzanon pheflomerlon' Light Hot Rolling CflndlilOnS and The present invention Wlll be more clear from the fol- Magnetic Characteristics lowmg examples' Light Hot Rolling Conditions Magnetic Characteristics EXAMPLE l 15 r R d H ld' w l ii Twelve silicon steel slabs (180mm thick) containing 255 R ihe l im w in Densiii B, 305% Si, 0.023% sol.Al, 0.0063% total N and unavoidable impurities were hot rolled to 2.25mm thickness by 600C 8 3 5 a continuous hot rolling mill and comparison tests were 1 I1) I L43 [.76 conducted under the following conditions shown in 20 850C Table 3. 10.0 3 |.l2 1.92

Table 3 Conditions of hot rolling and cooling Methods Conventional Inventive Conditions Reduction Final Rate 10% 10% Stand Speed 550 mpm 550 mpm Finishing Temperature 830 850C 830 850C Cooling Water cooling Water cooling Conditions started [m after started 40m after the outlet of the the outlet of the final stand final stand (about 1 see) (about 4 sec.) Coiling Not higher than Not higher than Temperature 500C 500C Then the hot rolled steel strips were cold rolled to 8 0.30mm thickness, subjected to decarburizing anneal- IGOOOC 9'5 3 p ing in wet hydrogen at 830C for 3 minutes and a high L79 As hot rolled L43 l.75

temperature annealing at 1,200C for hours to obtain grain-oriented electrical steel sheets.

The magnetic characteristics of the electrical steel sheets thus obtained are shown in Table 4.

Table 4 Magnetic Characteristics Cooling Conditions of Hot Rolled Strips Conventional Inventive Cooling started Cooling started Measurement vius done at l2 points (cm Epstein samples) As seen from the results shown in Table 4, the magnetic characteristics are remarkably improved by application of the present invention to the conventional continuous rolling hot rolling mill.

Units: Watt Loss W/Kg. Magnetic Flux Density Wit/m From the relation between the light hot rolling conditions and the magnetic characteristics shown in Table 5, it is clear that remarkable improvement of the magnetic characteristics can be obtained by application of the present invention to the hot rolled steel strips.

What is claimed is:

l. A process for producing a goss texture. grainoriented electrical steel sheet having excellent magnetic characteristics greater than I .85 Wb/m comprising hot rolling a silicon sheet material consisting essentially of not less than 2% silicon, 0.0! to 0.45% acid soluble aluminium, 0.003 to 0.0095% nitrogen with the balance being iron and unavoidable impurities, cold rolling the hot rolled material to a final sheet thickness. subjecting the cold rolled sheet to decarburization annealing and further subjecting the sheet to a final annealing in a temperature range where the secondary recrystallisation proceeds in the presence of aluminium nitride as it exists in the steel sheet thus produced, wherein the hot rolled sheet is subjected to a light hot working of 3 to 20% in a temperature range between steel material consists essentially of 2.0 to 4.5% silicon, 0.027 to 0.045% acid soluble aluminium, 0.0065 to 0.0095% nitrogen. and the light hot working is performed with a reduction rate between [0 and 16% at a temperature between 720 and 830C. 

1. A PROCESS FOR PRODUCING A GOSS TEXTURE GRAIN-ORIENTED ELECTRICAL STEEL SHEET HAVING EXCELLENT MAGNETIC CHARACTERISTICS GREATER THAN 1.85 WB/M2 COMPRISING HOT ROLLING A SILCON SHEET MATERIAL CONSISTING ESSENTIALLY OF NOT LESS THAN 21% SILICON, 0.01 TO 0.45% ACID SOLUBLE ALUMINIUM, 0.003 TO 0.0095% NITROGEN WITH THE BALANCE BEING IRON AND UNAVOIDABLE IMPURITIES, COLD ROLLING THE HOT ROLLED MATERIAL TO A FINAL SHEET THICKNESS SUBJECTING THE COLD ROLLED SHEET TO DECARBURIZATION ANNEALING AND FURTHER SUBJECTING THE SHEET TO A FINAL ANNEALING IN A TEMPERATURE RANGE WHERE THE SECONDARY RECRYSTALLISATION PROCEEDS IN THE PRESENCE OF ALUMINIUM NITRIDE AS IT EXISTS IN THE STEEL SHEET THUS PRODUCED, WHEREIN THE HOT ROLLED SHEET IS SUBJECTED TO A LIGHT HOT WORKING OF 3 TO 20% IN A TEMPERATURE RANGE BETWEEN 700* TO 900*C AND IS HELD IN THIS TEMPERATURE RANGE FOR NOT LESS THAN 2 SECONDS PRIOR TO THE COLD ROLLING TO PRECIPITATE ALUMINIUM NITRIDE WHICH IS EFFECTIVE FOR THE SECONDARY RECRYSTALLISATION AT THE STAGE OF FINAL ANNEALING.
 2. A process according to claim 1, wherein the silicon steel material consists essentially of 2.0 to 4.5% silicon, 0.027 to 0.045% acid soluble aluminium, 0.0065 to 0.0095% nitrogen, and the light hot working is performed with a reduction rate between 10 and 16% at a temperature between 720* and 830*C. 